The present invention relates to an actuator assembly and, more particularly, to such an assembly which provides axial movement of a ball screw between an extended and a retracted position, and a system for limiting the amount of torque applied to the assembly.
Actuator assemblies, in which a helically grooved ball screw is mounted for reciprocal axial movement in a ball nut, are used in many environments in which axial movement of a device is desired. The ball nut is secured against axial movement and is rotated to cause corresponding axial movement of the ball screw to an extended position. Actuators are used with many devices, such as, for example, gate valves or the like, which are attached to the ball screw for axial movement therewith. The ball nut can be rotated by fluid pressure as disclosed in U.S. Pat. No. 4,691,893; by an electrical motor as disclosed in U.S. Pat. No. 5,195,721; or in any other conventional manner.
In some of these assemblies, an inner race is driven, via a reduction gearing system, by the power source and is coupled to a driver by a wrap spring which functions as a clutch. The driver is connected, by a gear and pinion, to the ball nut for rotating same. A low power activating device, such as an electrical solenoid, or switch, is usually provided which, when activated by a relatively low power signal, engages a control module that latches to the driver and therefore couples the driver to the inner race by causing the wrap spring to couple the inner race to the driver. This, in turn, causes the ball nut to rotate and the ball screw to extend from the ball nut and move to its fully extended position.
Some of these arrangements employ a system for limiting the load on, or the amount of torque applied by, the power source. More particularly, in the event the above-described gears, as well as the components associated with the gears, the ball screw, the gate valve, or any device connected to the ball screw, become jammed, or are otherwise unable to rotate properly, the load on the power source is dramatically increased. In order to avoid damage to the power source, torque limiting systems have evolved. For example, some use a ball, or roller, that extends axially into a detent, or the like, in a rotating member for coupling the power source to the gear train of the actuator. However, these arrangements are prone to rather excessive axial vibration that can cause other sensitive parts of the actuator to malfunction. Other types of torque limiting systems utilize a frictional engagement for coupling the power source to the gear train of the actuator. However, the mating parts of these devices heat up to the extent that their operation is impaired.
Therefore, what is needed is an actuator assembly and a torque limiting system for the assembly which couples the power source of the actuator to its gear train, yet decouples same in response to excessive loads being applied to the power source, while minimizing axial vibration and excessive heating of the components of the system.